URGENT!! How many moles are in 8mL of 3 percent H2O2?

1000mL contains 0.88 mol H2O2
8mL will contain 8mL /1000mL/L *0.88mol/L = 0.00704mol in 8mL

Method 1 :

Mass of H₂O₂ in 8 mL of 3%(w/v) H₂O₂ solution = 8 × 3% g = 0.24 g

Molar mass of H₂O₂ = (1.0×2 + 16.0×2) g/mol = 34.0 g/mol
Mass of H₂O₂ in 8 mL of 3% H₂O₂ solution = (0.24 g) / (34.0 g/mol) = 0.00706 mol ≈ 0.007 mol (to 1 sig. fig.)


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Method 2 : (You have to show that 3% H₂O₂ solution is 0.88 mol/L.)

Mass of H₂O₂ in 1000 mL (1 L) of 3%(w/v) H₂O₂ solution = 1000 × 3% g = 30 g

Molar mass of H₂O₂ = (1.0×2 + 16.0×2) g/mol = 34.0 g/mol
Molarity of 3% H₂O₂ solution = (30/34.0 mol) / (1 L) = 0.882 mol/L = (0.882 mol) / (1000 mL)

Mass of H₂O₂ in 8 mL of 3% H₂O₂ solution = [(0.882 mol) / (1000 mL)] × (8 mL) = 0.00706 mol ≈ 0.007 mol (to 1 sig. fig.)

There are 0.88 moles in 1 lit ie 1000 ml. So there will be:

0.88 x 8/1000 mol in 8 ml.

first of all... %'s and ppm's are ambiguous terms. I always recommend you get those defined (by your instructor) if you are not absolutely sure what they mean for your particular problem
.. 3% could mean.. 3% w/w.... i.e.. 3g H2O2 / 100g solution x 100%
.. 3% could mean.. 3% w/v... . i.e.. 3g H2O2 / 100mL solution x 100%
.. 3lbs / 100gal x 100%
.. 3 g / 100L x 100%
.. etc.
get it? without knowing the type of percent (w/v... v/v... w/w...etc) and the units (g/mL, lb/gal, g/g, etc), 3% could literally mean anything. If you are not absolutely sure.... get it defined!

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for this problem, let's assume it means 3g H2O2 / 100mL solution

why am I assuming that? because you wrote 0.88 mol/L.... see this calc.
.. ... 3g H2O2.. ... ... .1 mol H2O2.. ..1000mL
---- ----- ---- ---- --- x ----- ------ ----- x ----- ----- = 0.882 mol / L = 0.882 M... .<--- your 0.88 mol/L
.. 100mL solution.. .. 34.02g H2O2.. . ..1 L

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that out of the way... let's use dimensional analysis to solve this problem

.. 8 mL... ... . 3g H2O2... . 1 mol H2O2
----- ----- x ------ ---- ----- x ----- ----- ----- = 0.00705 mol H2O2.... YOU get to figure out the sig figs
.. .. 1.. .. .. .... 100mL... ... 34.02g H2O2